Versatile Chemical Transformations of Benzoxazole Based Ligands on Complexation with 3d-Metal Ions

Two benzoxazoles derivative ligands were synthesized from the condensation of 3,5-di-tert-butyl-o-benzoquinone (DTBBQ) with ethanolamine or 1,3-diamino-2-hydroxypropane in methanol. Condensation of DTBBQ with ethanolamine gives the expected 5,7-di-tert-butyl-2-methylenhydroxylbenzoxazole (HL1) while with 1,3-diamino-2-hydroxypropane it gives (2-hydroxyethyl-2-{2,4-bis­(1,1-dimethylethyl)-1-phenol-6 amino}-2­{5,7-di-tert-butyl-benzoxazole}) (H2L2) with only one benzoxazole ring instead of the symmetric bis-benzoxazole derivative. The structure of HL1 and H2L2 were confirmed by NMR-spectroscopy and X-ray diffraction on a single crystal for HL1. The reaction of HL1 with CuCl2 gives a mononuclear [CuII(HL1)2Cl2] (1) complex for which the crystal structure shows that HL1 is preserved. In contrast, upon reaction with nickel­(II), cobalt­(II), and manganese­(II) H2L2 is further oxidized and transformed in new ligands HL3 in mononuclear complexes [MII(L3)2] (M = NiII (2); M = CoII (3)) and H2L4 in tetranuclear complex [MnII4(HL4)4Cl4] (4) as found from the crystal structures of complexes 2–4. Electrochemical studies for complexes 2 and 3 evidence complicated redox properties. [MnII4(HL4)4Cl4] (4) has a cubane-like structure with a “4 + 2” fashion The magnetic susceptibility of 4 is well fitted considering one Mn---Mn interaction Ja(MnII-MnII) = −0.50(1) cm–1 with g = 2.00(7).